In the field of the optical network serving as the base in the Internet communication network and the like, the optical MEMS (Micro Electro Mechanical System) technique attracts attention as a technique that realizes a multichannel, wavelength division multiplex (WDM), low-cost system, and an optical switch has been developed using this technique (for example, see Japanese Patent Laid-Open No. 2003-57575). One of the most characteristic constituent components of the MEMS type optical switch is a mirror array. In the mirror array, a plurality of mirror devices are arranged two-dimensionally to form a matrix. FIG. 21 shows an example of a conventional mirror device comprising one mirror to serve as one constituent unit of a mirror array.
A mirror device 7000 has a structure in which a mirror substrate 8000 having a mirror 830 and an electrode substrate 9000 having electrodes 940a to 940d are arranged parallel to each other to oppose each other.
The mirror substrate 8000 comprises a plate-like frame portion 810 having an opening which is almost circular when seen from the top, a gimbal 820 disposed in the opening of the frame portion 810 by a pair of gimbal connectors 811a and 811b and having an opening which is almost circular when seen from the top, and the mirror 830 which is disposed in the opening of the gimbal 820 by a pair of mirror connectors 821a and 821b and almost circular when seen from the top. A frame-like member 840 surrounding the gimbal 820 and mirror 830 is arranged on the upper surface of the frame portion 810.
The electrode substrate 9000 has a plate-like base 910 and a conical projection 920 which projects from the surface (upper surface) of the base 910 and is formed at a position opposing the mirror 830 of the mirror substrate 8000. The four fan-shaped electrodes 940a to 940d are formed on the outer surface of the projection 920 and the upper surface of the base 910 to fall within a circle concentric with the mirror 830 of the opposing mirror substrate 8000. A pair of protrusions 960a and 960b which line up to sandwich the projection 920 are formed on the upper surface of the base 910. Furthermore, interconnections 970 are formed between the projection 920 and protrusion 960a and between the projection 920 and protrusion 960b on the upper surface of the base 910. The interconnections 970 are connected to the electrodes 940a to 940d through lines 941a to 940d. 
The mirror substrate 8000 and electrode substrate 9000 as described above constitute the mirror device 7000 as the lower surface of the frame portion 810 is bonded to the upper surfaces of the protrusions 960a and 960b such that the mirror 830 opposes the electrodes 940a to 940d that oppose it.
In the mirror device 7000, voltages are separately applied to the electrodes 940a to 940d through the interconnections 970, so that electric fields formed by the potential differences between the mirror 830 and the electrodes 940a to 940d apply electrostatic attracting forces to the mirror 830. This elastically deforms the gimbal connectors 811a and 811b and mirror connectors 821a and 821b to tilt the mirror 830 through an angle of several degrees. This operation can be described as follows by referring to FIG. 22. When no voltage is applied to the electrodes 940a to 940d, the mirror 830 is in a state (to be referred to as an initial position hereinafter) of almost parallel to the electrode substrate 9000, as indicated by a solid line in FIG. 22. In this state, when a voltage is applied to, e.g., the electrode 940a, the gimbal 820 and mirror 830 pivot about a pivot axis extending through the gimbal connectors 811a and 811b and a pivot axis extending through the mirror connectors 821a and 821b, respectively, to tilt as indicated by a broken line in FIG. 22. To effect this tilting operation smoothly, springs, that is, the gimbal connectors 811a and 811b and mirror connectors 821a and 821b employ a structure that enables them to pivot easily in a direction (to be referred to as a direction R hereinafter) about an axis (to be referred to as a pivot axis or X-axis hereinafter) that connects one connecting point (the stationary frame 810 or gimbal 820) and the other connecting point (the gimbal 820 or mirror 830).
For example, as shown in FIGS. 21 and 23, the conventional mirror device 7000 employs a spring with a serpentine-shaped structure that flexes repeatedly in a direction perpendicular to the X-axis direction. Among parameters representing the serpentine structure, the length in the direction (to be referred to as the Z-axis direction hereinafter) perpendicular to the X-axis and the major surface of the mirror 830, the length in the direction of the pivot axis, the number of folds, the length in the direction (to be referred to as the Y-axis direction hereinafter) perpendicular to the X-axis and Z-axis, the gaps among the folding portions, and the like are the parameters that determine the characteristics such as the spring constant of the spring. By appropriately setting these parameters, the elastic characteristics, particularly spring constants in the direction R, of the gimbal connectors 811a and 811b and mirror connectors 821a and 821b take desired values.